1. Field of the Invention
The present invention relates to improved foamed cement compositions and methods of using the compositions for cementing wells.
2. Description of the Prior Art
Foamed hydraulic cement compositions are utilized in various applications including in surface construction and in subterranean well completion and remedial operations. For example, foamed cement compositions are used in primary well cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In surface construction applications, the cement compositions utilized must often be lightweight. In well applications, the cement compositions must also often be lightweight to prevent excessive hydrostatic pressure from being exerted on subterranean formations penetrated by the well bore whereby the formations are unintentionally fractured. In addition to being lightweight, a foamed cement composition contains compressed gas which improves the ability of the composition to maintain pressure and prevent the flow of formation fluids into and through the cement composition during its transition time, i.e., the time during which the cement composition changes from a true fluid to a hard set mass. Foamed cement compositions are also advantageous because they have low fluid loss properties.
The cement compositions utilized for cementing pipe strings in well bores must develop high strength after setting and also should have sufficient resiliency, i.e., elasticity and ductility, to resist the loss of the cement bond between the pipe and formation. That is, the set cement composition in the annulus between a pipe string and the walls of a well bore often fail due to pipe movements which cause shear and/or compressional stresses to be exerted on the set cement. Such stress conditions are commonly the result of relatively high fluid pressures and/or temperatures inside the cemented pipe string during testing, perforating, fluid injection or fluid production. The high internal pipe pressure and/or temperature results in the expansion of the pipe string both radially and longitudinally, which places stresses on the cement sheath causing it to crack or causing the bonds between the cement composition and the exterior surfaces of the pipe or formation, or both, to fail which allows leakage of formation fluids into or through the well bore.
Stress conditions also result from high pressures which occur inside the cement sheath due to the thermal expansion of fluids trapped within the cement sheath. This condition often occurs as a result of high temperature differentials created during the injection or production of high temperature fluids through the well bore, e.g., wells subjected to steam recovery or the production of hot formation fluids from high temperature formations. Typically, the pressure of the trapped fluids exceeds the collapse pressure of the cement and pipe causing leaks and bond failure. Other compressional stress conditions occur as a result of outside forces exerted on the cement sheath due to formation shifting, overburden pressures, subsidence and/or tectonic creep.
Thus, there are needs for improved cement compositions and methods of utilizing the compositions in wells whereby after setting, the cement compositions form resilient solid masses which have high compressive, tensile and bond strengths sufficient to withstand the above described stresses without failure.